U.S. Pat. No. 3,722,506 discloses a spirometer in which a seal is formed between the piston and the inner surface of the cylinder by a rolling seal membrane interposed between the two members. A similar construction, illustrative of the prior art, is shown herein in FIG. 1. The spirometer, intended for measuring the breathing volume or breathing rate of human subjects, includes a cylindrical outer casing 10 having a tubular inlet 11 at one end for the entry (and discharge) of expired air. Air entering the cylindrical casing displaces piston 12 towards the forwardly-extended position shown in the drawing, and the extent of displacement is recorded by suitable recording means 13 connected to piston shaft 14. A U-shaped sealing membrane 15 is interposed between the outer surface of the piston and the inner surface of the cylindrical casing with one edge 15a of the membrane being sealed to the piston and the other edge 15b being sealed to the casing.
FIG. 1 shows the piston at or near maximum extension although, if it were not for rear end wall 16 that acts as a stop, the piston would be free to travel additional distance forwardly until membrane 15 became stretched along the inside wall of the casing as shown hypothetically in FIG. 2. In such a hypothetical situation, the stroke and volume of the spirometer would be increased, resulting in a spirometer of greater capacity for any given diameter. However, such an extended-stroke spirometer is not known in the prior art. An important reason is believed to lie in the fact that if the piston stroke were extended as shown in FIG. 1A, the membrane would no longer be supported by the piston and would be free to collapse into the chamber as shown in hypothetical FIG. 1B. The possibilities of such collapse would be increased should ambient pressure exceed the pressure within the chamber. Thus, should internal pressure drop off at the end of a forceful discharge of expired air into the chamber, ambient pressure on the opposite side of the membrane would exert a force in the direction of arrow 17, causing the membrane 15 to collapse or bulge into the chamber and interfere with return movement of the piston (FIG. 1B).
One aspect of this invention lies in recognizing and understanding the problem described above and the functional reasons for it; another aspect lies in the discovery that such problem may be overcome by an uncomplicated and highly effective arrangement of two rolling membranes disposed so that one acts as a buffer to protect the other against pressures that would otherwise promote membrane collapse. The result is a spirometer of expanded stroke and volume for a given diameter or, conversely, a unit of smaller diameter, with lower inertia and greater sensitivity, for a given volumetric capacity.
The reversely-folded elastomeric membranes have their respective edges secured to the outer surface of the piston and the inner surface of the cylinder so that the rounded folds of the membranes face each other. A limited space is provided between the external surfaces of the folds, and means may be provided for reducing pressure in that space to insure against the possibility of membrane collapse and to improve the rolling action of the membranes as the piston extends and retracts. Since the provision of reduced pressure within the inter-membrane space tends to cause a slight stretching and an increase in roundness of curvature of the membranes in the area of the folds, greater linearity of response tends to result. Stated differently, initial resistance to piston movement that might otherwise be caused by sharpness or abruptness of the folds, and by the forces necessary to alter their configuration, is reduced because the lower pressure in the inter-membrane space causes a slight stretching and enlarging of the folds and gives them a smoother and more uniformly rounded contour which in turn produces a more effective low-resistance rolling action of the sealing members or membranes.
The double rolling seal of this invention not only permits greater piston stroke and spirometer capacity for a given piston diameter, but also produces smoother and more uniform rolling sealing action, enhances linearity of response, and promotes self-alignment of the piston. Return of the piston to its starting or "zero" position is effectively achieved by a serpentine leaf spring connected to the piston and the spirometer housing. The spring, although similar in appearance to the leaf spring disclosed in U.S. Pat. No. 3,086,515, is mounted and arranged to perform a different function and achieve a different result. Specifically, the serpentine spring as used herein is arranged so that its piston-returning force is at its greatest when the piston is fully retracted (i.e., in its zero position) and progressively diminishes as the piston is extended. When the piston reaches the forward limit of its travel, the spring is close to a neutral condition with the return force exerted by the spring being at a minimum for the spirometer system.
Other features, objects, and advantages of the invention will become apparent from the specification and drawings.